1. Field of the Invention
The present invention relates to a phosphor for a low-voltage electron beam and to a vacuum fluorescent display apparatus in which the phosphor is used.
2. Description of the Related Art
Cadmium-sulfide-based phosphors have conventionally been used as a red phosphor to be excited by a low-voltage electron beam. However, cadmium has been restricted from use due to environmental problems because it is a hazardous element, and there is a need for a red phosphor that does not include cadmium. SrTiO3:Pr, CaTiO3:Pr, and the like have been conventionally known as phosphors that do not include cadmium, but these phosphors are problematic in that they suffer from severe brightness degradation, short phosphor service life, and poor initial brightness.
Accordingly, there have been proposed (1) methods for forming a solid solution of a second additive as an activator in a phosphor matrix as a method for improving the initial brightness of a CaTiO3:Pr,M phosphor (Japanese Laid-open Patent Application Nos. 2005-281507, 2005-281508, 8-85788, and 2009-298997); (2) methods for forming a metal oxide layer on the surface of a phosphor as a method for preventing brightness degradation in a phosphor having a matrix composed of an alkaline-earth metal and an oxide (Japanese Laid-open Patent Application No. 8-283709); (3) methods for depositing a tin compound, which is changed into an oxide by heat treatment, onto the surface of the phosphor particles as a method for improving the service life characteristics without a considerable reduction in the initial brightness of the phosphor (Japanese Laid-open Patent Application No. 2006-335898); and (4) methods for increasing brightness without obstructing the emission of light from other rare earth elements by further adding any among Gd, La, and Y as a third additive to the phosphor matrix in a CaTiO3:Pr,Zn,Li phosphor (Japanese Laid-open Patent Application No. 2009-242735).